Naturally occurring somatotropins are polypeptides, the amino acid sequences of which for a number of vertebrate species have been reported. These include bovine (Miller et al., 1980, J. Biol. Chem, 255, 7251) porcine (Seeburg et al., 1983, DNA 2: 37), human (U.S. Pat. No. 3,853,832; Martial et al., Science, 205: 602-617) and various piscine somatotropins (e.g.: Sekine et al., 1985. Proc. Natl. Acad. Sci. (USA), 82: 4306-4310; Agellon et al., 1988, Proc. Natl. Acad. Sci. (USA), 85: 5136-5140; U.S. Pat. Nos. 4,689,402 and 4,894,362). In general, somatotropins isolated from different species display a high degree of amino acid sequence identity (Chang et al., 1992, Gen. and Comp. Endocrin. 87: 385-393). Analogs of somatotropins are also known. European Patent Application 103 395, for example discloses bovine somatotropin analogs. These analogs typically relate to the insertion, addition or deletion of nucleotides of the somatotropin gene thereby creating a protein different from the naturally occurring somatotropin.
The preparation of somatotropins is well known in the art. Bovine somatotropin, for example can be prepared by extraction from the pituitary tissue, (Li et al., 1954, J. Biol. Chem. 211: 55 and U.S. Pat. No. 4,371,462). Somatotropins can also be prepared by production in genetically engineered microorganisms, such as Escherichia coli containing recombinant DNA which encodes a somatotropin polypeptide (e.g. Seeburg et al., 1978, Nature). U.S. Pat. No. 4,443,549 discloses a method for producing bovine somatotropin in yeast cells. Methods for high yield bovine somatotropin production in microorganisms are disclosed in U.S. Pat. Nos. 5,240,837 and 5,489,529.
Similarly, the preparation of human somatotropin is known. For example U.S. Pat. No. 5,637,495 and Mukhija et al. (Gene 165: 303-306) teach the production of human growth hormone in E. coli.
Fish somatotropins have also been produced in a variety of microorganisms. U.S. Pat. No. 5,270,180 for example discloses a method for the production of salmon growth hormone in E. coli and yeast and in U.S. Pat. No. 4,894,362 a microbial production system for eel growth hormone is disclosed.
The low costs associated with growing plants, make plants an attractive host for the production of somatotropins. To the best of the present inventors knowledge only one attempt has been reported to produce a somatotropin in plants. Bosch et al. (Transgenic Research, 1994, 3: 304-310) expressed a trout growth hormone in the leaves of transgenic tobacco plants, however they were unsuccessful in accumulating somatotropin in seeds.
Although methods for producing somatotropins are well known to skilled artisans, the existing methods are relatively expensive, especially when large production volumes are required. Accordingly there is a need in the art for additional economical production methods of somatotropin.